The present invention relates to a protective device for a power supply system, and particularly to a grounding protective relay which compensates for zero phase circulation current between lines and zero phase vertical voltage drops generated by induction in a common suspension multi-line system in which a common suspension multi-line high resistance grounding system (including a Peterzen Coil grounding system) has a T-branch load between two parallel lines.
In electrical transmission systems with very high voltage transmission lines and common suspension high resistance grounding systems, because the very high voltage transmission lines are arranged in non-crossover suspension, and reverse phase distribution, the flow of current in the very high voltage transmission line induces zero phase circulation currents between lines (henceforth referred to simply as zero phase circulation currents) and zero phase vertical voltage drops, and for this reason the grounding protective relay operates erroneously or fails erroneously to operate. Because of this, it is necessary to compensate for zero phase circulation currents and zero phase vertical voltage drops.
The present invention is a result of considering the above mentioned points, and it is the object of the present invention to provide a grounding protective relay which makes the influence of zero phase circulation currents and zero phase vertical voltage drops as small as possible, with the capability of accurately detecting grounding fault.